20b.rb

#
# use this to print at the end of part 1
#
# (0...input.dim).each do |r|
#   lines = []
#   (0...input.dim).each do |c|
#     tile = board[[ r, c ]]
#     tile[1..-2].each.with_index do |line, index|
#       (lines[index] ||= '') << line[1..-2].join
#     end
#   end
#   puts lines.join("\n")
# end
#

data = DATA.lines.map(&:chomp).map(&:chars)
monster = MONSTER.lines.map(&:chomp).map(&:chars)

def check(data, monster)
  match = false
  (0...data.length - monster.length).each do |r|
    (0...data.first.length).each do |c|
      # is there a monster here?
      copy = data.map(&:dup)
      n = 0
      (0...monster.length).each do |r2|
        (0...monster.first.length).each do |c2|
          if monster[r2][c2] == '#' && copy[r + r2][c + c2] == '#'
            copy[r + r2][c + c2] = 'O'
            n += 1
          end
        end
      end

      # if we found a monster, make sure we use the new state from here on out
      if n == 15
        match = true
        data = copy
      end
    end
  end

  if match
    p data.map(&:join).join.chars.count { |i| i == '#' }
    exit
  end
end

2.times do
  # original
  check(data, monster)
  # rotate 3 times
  3.times { check(data = data.transpose.map(&:reverse), monster) }
  # flip
  data = data.reverse
end